In a slight departure from Steam-powered amusements, Two Hour Wargames' 5150 Battalion Commander is now on sale. Command a Sci-Fi armor battalion in planetary conquest. Battalion Commander is the latest entry in Two Hour Wargames Science Fiction universe, a stand-alone game that builds on and complements the other titles in the series.
To quickly get right into massive mecha battles, you can use Armor Grid's extensive collection of print-and-play 15mm miniatures. Infantry, mechs, vehicles and aircraft in multiple color schemes are all there for you to command. Build an army as large as your printer can handle.
Armor Grid's own game, Mech Attack, is a best seller on Wargame Vault. Due to various circumstances I lean towards games with a solo option, like the 5150 series.